1. Field of the Invention
The present invention relates to a method for recording data in an optical recording medium, an apparatus for recording data in an optical recording medium and an optical recording medium, and particularly, to a method for recording data in a write-once type optical recording medium, an apparatus for recording data in a write-once type optical recording medium, and a write-once type optical recording medium.
2. Description of the Related Art
Optical recording media such as the CD, DVD and the like have been widely used as recording media for recording digital data. These optical recording media can be roughly classified into optical recording media such as the CD-ROM and the DVD-ROM that do not enable writing and rewriting of data (ROM type optical recording media), optical recording media such as the CD-R and DVD-R that enable writing but not rewriting of data (write-once type optical recording media), and optical recording media such as the CD-RW and DVD-RW that enable rewriting of data (data rewritable type optical recording media).
As well known in the art, data are generally recorded in a ROM type optical recording medium using pre-pits formed in a substrate in the manufacturing process thereof, while in a data rewritable type optical recording medium a phase change material is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by phase change of the phase change material.
On the other hand, in a write-once type optical recording medium, an organic dye such as a cyanine dye, phthalocyanine dye or azo dye is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by chemical change of the organic dye, or chemical change and physical change of the organic dye.
Further, there is known a write-once type recording medium formed by laminating two recording layers (See Japanese Patent Application Laid Open No. 62-204442, for example) and in this optical recording medium, data are recorded therein by projecting a laser beam thereon and mixing elements contained in the two recording layers to form a region whose optical characteristic differs from those of regions therearound.
In this specification, in the case where an optical recording medium includes a recording layer containing an organic dye, a region in which an organic dye chemically changes or chemically and physically changes upon being irradiated with a laser beam is referred to as “a recording mark” and in the case where an optical recording medium includes two recording layers each containing an inorganic element as a primary component, a region in which the inorganic elements contained in the two recording layers as a primary component are mixed upon being irradiated with a laser beam is referred to as “a recording mark”.
An optimum method for modulating the power of a laser beam projected onto an optical recording medium for recording data therein is generally called “a pulse train pattern” or “recording strategy”.
FIG. 10 is a diagram showing a typical pulse train pattern used for recording data in a CD-R including a recording layer containing an organic dye and shows a pulse train pattern for recording 3T to 11T signals in the EFM Modulation Code.
As shown in FIG. 10, in the case where data are to be recorded in a CD-R, a recording pulse having a width corresponding to the length of a recording mark M to be formed is generally employed (See Japanese Patent Application Laid Open No. 2000-187842, for example).
More specifically, the power of a laser beam is fixed to a bottom power Pb when the laser beam is projected onto a blank region in which no recording mark M is formed and fixed to a recording power Pw when the laser beam is projected onto a region in which a recording mark M is to be formed. As a result, an organic dye contained in a recording layer is decomposed or degraded at a region in which a recording mark M is to be formed and the region is physically deformed, thereby forming a recording mark M therein. Here, the ratio of the shortest blank region interval (3T) to the linear recording velocity (shortest blank region interval/linear recording velocity) is about 700 nsec at a 1× linear recording velocity of a CD-R.
FIG. 11 is a diagram showing a typical pulse train pattern used for recording data in a DVD-R including a recording layer containing an organic dye and shows a pulse train pattern for recording a 7T signal in the 8/16 Modulation Code.
Since data are recorded in a DVD-R at a higher linear recording velocity than when recording data in a CD-R, unlike the case of recording data in a CD-R, it is difficult to form a recording mark having a good shape using a recording pulse having a width corresponding to the length of the recording mark M to be formed.
Therefore, data are recorded in a DVD-R using a pulse train in which, as shown in FIG. 11, the recording pulse is divided into a number of divided pulses corresponding to the length of the recording mark M to be formed.
More specifically, in the case of recording an nT signal where n is an integer equal to or larger than 3 and equal to or smaller than 11 or 14 in the 8/16 Modulation Code, (n−2) divided pulses are employed and the power of the laser beam is set to a recording power Pw at the peak of each of the divided pulses and set to a bottom power Pb at the other portions of the pulse. In this specification, the thus constituted pulse train pattern is referred to as “a basic pulse train pattern”. Here, the ratio of the shortest blank region interval (3T) to the linear recording velocity (shortest blank region interval/linear recording velocity) is about 115 nsec at a 1× linear recording velocity of a DVD-R.
However, in a study done by the inventors of the present invention, it was found that as the ratio of the shortest blank region interval to the linear recording velocity (shortest blank region interval/linear recording velocity) decreased, when the basic pulse train pattern was employed for forming a short recording mark M, the edge portion of the recording mark M on the downstream side thereof with respect to the moving direction of the laser beam (hereinafter referred to as a “rear edge portion of a recording mark M”) tended to shift toward the moving direction of the laser beam, whereby the recording mark M became longer than the desired length and jitter of the reproduced signal became worse.
It is found that this phenomenon becomes pronounced in the case where the ratio of the shortest blank region interval (3T) to the linear recording velocity (shortest blank region interval/linear recording velocity) is equal to or smaller than 40 nsec and that in the case where the ratio of the shortest blank region interval (3T) to the linear recording velocity (shortest blank region interval/linear recording velocity) is further smaller and equal to or smaller than 20 nsec, the edge portion of a small recording mark M on the upstream side thereof with respect to the moving direction of the laser beam (hereinafter referred to as a “front edge portion of the recording mark M”) also tends to be shifted toward the direction opposite to the moving direction of the laser beam, whereby the recording mark M becomes much longer than the desired length and jitter of the reproduced signal becomes still worse.
It is reasonable to assume that the reason why the rear edge portion of a recording mark is shifted toward the moving direction of the laser beam is because the recording layer is physically and/or chemically changed by heat generated by the laser beam having the recording power Pw projected thereonto for forming the recording mark even at a region downstream of the rear edge portion of the recording mark with respect to the moving direction of the laser beam. On the other hand, it can be assumed that the reason why the front edge portion of a recording mark is shifted to toward the direction opposite to the moving direction of the laser beam is because the temperature of a track is increased by thermal interference between neighboring recording marks, whereby the temperature of a region irradiated with the laser beam having the recording power Pw projected thereonto for forming the recording mark becomes too high.
In the case where the length of a recording mark becomes longer than a predetermined length, since jitter of the reproduced signal becomes markedly worse, it is necessary to prevent a recording mark from becoming longer than a predetermined length.
It might be considered possible to prevent a recording mark from becoming longer than a predetermined length by lowering the recording power Pw of the laser beam, thereby decreasing the amount of heat applied to the recording layer when the recording mark is formed.
However, in the case where the recording power Pw of the laser beam is lowered, the width of a recording mark becomes thin and the C/N ratio (carrier/noise ratio) of the signal is therefore lowered.
Another solution that might be considered is to shorten the period during which the power of the laser beam is set to the recording power Pw, namely, the width of the pulse to lower the total amount of heat applied to the recording layer when a recording mark is to be formed. However, since the modulation rate of the power of a laser beam has limitations, the pulse width of the laser beam sometimes cannot be set to the desired width in the case where the linear recording velocity is particularly high.
Thus it was found that in the case where the basic pulse train pattern is employed, it becomes increasingly difficult to obtain a signal having good signal characteristics as the ratio of the shortest blank region interval to the linear recording velocity (shortest blank region interval/linear recording velocity) decreases.
The above mentioned problems are particularly pronounced in a write-once type optical recording medium in which a recording mark is formed by projecting a laser beam thereonto to generate heat and mixing elements contained in a plurality of recording layers by the heat.
On the other hand, a next-generation type optical recording medium that offers improved recording density and has an extremely high data transfer rate has been recently proposed.
In such a next-generation type optical recording medium, the achievement of increased recording capacity and extremely high data transfer rate inevitably requires the diameter of the laser beam spot used to record and reproduce data to be reduced to a very small size.
In order to reduce the laser beam spot diameter, it is necessary to increase the numerical aperture of the objective lens for condensing the laser beam to 0.7 or more, for example, to about 0.85, and to shorten the wavelength of the laser beam to 450 nm or less, for example, to about 400 nm.
In addition, since data are recorded at an extremely high linear recording velocity in the next-generation type optical recording medium, it is required to increase the recording power Pw of a laser beam. However, since a semiconductor laser device having high output is expensive and the service life of a semiconductor laser device decreases as the recording power Pw of the laser beam is set higher, it is preferable to record data using a laser beam whose recording power Pw is as low as possible.
In order to record data in a next-generation type optical recording medium using a laser beam whose recording power Pw is as low as possible, it is effective to set the bottom power Pb of the laser beam high so as to augment the heating of the recording layer by the laser beam having the recording power Pw.
However, if the bottom power Pb of the laser beam is set high, the rear edge portion and front edge portion of a recording mark are shifted more greatly and the recording layer is sometimes physically and/or chemically changed even at a blank region, whereby data cannot be recorded therein.